As is known, filter cigarettes are usually manufactured by connecting together, by means of adhesive wrappers, pairs of axially aligned cigarettes, between which is positioned a double length filter plug; the assemblies or units thus obtained are then severed midway of the plug so that each assembly thus yields two filter cigarettes.
The wrappers which join the two cigarette bodies are successively severed or cut in turn from a continuously advanced web of paper material, having imitation cork on one side and coated with adhesive on the other. The cutting device comprises two contrarotating drums with parallel axes and each respectively provided with a plurality of co-operating blade means between which the web is advanced. One of the drums is also provided with means, usually suction means, designed to maintain the non-adhesive side of the web in the feed path between the blades, adhering to its external cylindrical surface in such a manner as to permit the same drum to path beneath the web and keep it adherent to its external surface while entraining it as a function of the feed speed of the same web, both during the feeding operation and during the severing or cutting operation. The drum holds the wrappers steady as they are successively cut from the web in order to space them out and apply them successively thus spaced out tangentially by adhesion of their adhesive surface to the above mentioned cigarette filter units. The latter are conveyed in likewise equidistant rows by a grooved drum provided in contrarotating relationship with the holding drum (see for example GB Patent Nos. 776,777 and 986,454).
Recent developments in these devices with contrarotating severing or cutting means fitted in automatic high speed filter tipping machines, as for example those according to British Patent Application GB 2066722A of 21st Nov. 1980, provide for the severing of the web, by means of knives with cutting edges contrarotating at different peripheral speeds with a grazing, severing or cutting action resulting from severing forces which are essentially radial.
One problem with such knives is that they operate well only at a given temperature. When, because of the heating of one of the knife carrying drums for the recovery of the adhesive coating of the wrappers, the knives are caused to heat up further, they expand radially outwardly; this results in their no longer being able to cut tangentially at successive points with a shearing action or with a striking action. British Patent Application 2066722A, for example, proposes to solve the problem of severing the wrappers in various ways, such as that of severing or cutting by pressure or by conventional tangential contact at successive points of the cutting edges of the knives contrarotating at different peripheral speeds (cutting action by shearing). This arrangement provides a co-operating device with contrarotating knives with a grazing action at successive points of the cutting edge of the higher speed knives against the cutting edge of the lower speed knife. (See also U.S. Pat. No. 3,247,746)
Essentially, then, while severing by means of conventional tangential contact at successive points of the cutting edges at different peripheral speeds, is carried out by a succession of severing or cutting strokes which may be defined as essentially tangential, severing in accordance with recent developments by means of the grazing action proposed in the above mentioned British Patent Application GB 2066722A is carried out rather by a succession of severing or cutting forces which may be defined as essentially radial with a scoring effect.
Applicant has found that the lack of success of such automatic high speed filter tipping machines with knives operating at the respective cutting edges contrarotating at different peripheral speeds with tangential reciprocal contact at successive points is not to be ascribed, as maintained in the Patent Application GB 2,066,722A, to the overheating and consequent radial expansion of the same knives, but rather to the incorrect reciprocation of their operative positions on their respective drums. I have found that if such an incorrect position allows operation at low speeds, it does not allow such good operation at higher speeds, thus causing further problems, such as, for example, the cutting time, this latter being a function of the speed, the operative angle of contrarotation of the knives themselves, between which the said cutting must be carried out, and the position of the said operative angle in relation to the tangential point of the plane traversed by the respective axes of contrarotation of the aforementioned cutting edges.
Above all, this is confirmed by the contents of U.S. Pat. No. 1,867,884 of 18th June 1931, which provides a device of the abovementioned type with knives contrarotating at different peripheral speeds in order to cut from a continuous web of serviettes, handkerchiefs, nappies and similar paper articles, defined at that time (more than fifty years ago) as at high speed, in which the knife at the higher peripheral speed is positioned at the cutting edge or border of the rectilinear path in a plane parallel to the plane traversed by the axis of rotation of the respective support drums and in an oblique or inclined position in the said plane with respect to the said axis in such a way that the marginal edge of the said lowermost rectilinear cutting edge, with respect to the direction of its rotational movement, results in a distance from the said axis corresponding to a smaller radius that that of the distance from the said axis to its other uppermost marginal edge.
With a similar arrangement of the said knife with an inclined rectilinear cutting edge with a higher peripheral speed, the operative cutting angle is, in the majority of cases, displaced completely below or completely above the plane traversed by the axes of contrarotation of the two knife-carrying drums according to the inclination of the said plane with respect to vertical, with a notable projection of the abovementioned upper edge of the said knife over the cylindrical surfaces of the respective drums, in order to be able, with regard to operative contact below or, respectively, above the said plane, to follow the rectilinear cutting edge of the other knife along the corresponding circumferential path of rotation, so that it is absolutely necessary to maintain the said knife with its rectilinear cutting blade inclined in an elastically yielding manner so as to allow the said cutting blade to follow a course according to a helicoidal spiral during the cutting action, even though at a very slow rate.
Another disadvantage of all the cutting devices hitherto known of the type defined above having a suction drum intended to maintain the web to be cut adhering thereto by sliding over its external surface, is that the section of the web or wrapper separated from the web, at a certain point in the cutting action, resists separation from the same when the as yet uncut section no longer slides with respect to the suction drum, and is completely held in adhesion to the same suction drum from which it has been drawn and is not severed by successive cuts along this latter severing section, the relative marginal edges being in an inclined discontinuous or irregularly jagged course, detrimental to the quality of the finished product.